The great penetrating power of the neutron makes neutron radiation a particularly dangerous form of ionizing radiation. In fact, it is likely that neutron radiation may constitute a major portion of a radiation worker""s annual radiation dose. A high dose of neutron radiation received in a matter of seconds can result in death. Even a fairly low dose can result in cancer or genetic damage to the recipient. For a worker to be safe, the dosage of neutron radiation must be regulated carefully and held below regulatory limits. To accomplish this, a radiation worker must utilize accurate and careful monitoring whenever neutron radiation is a possible hazard.
During the preceding 20 years, the locations where neutron radiation hazards exist have proliferated because of the increase in nuclear reactors and accelerators, radiotherapy facilities, plutonium-processing facilities, fusion research and isotopic sources. To attain a proper assessment of the neutron radiation hazards, and an accurate measurement of radiation worker dosage, a precise and sensitive neutron meter, having a wide energy range is required.
The prior art neutron rem meters not only suffer from limited sensitivity and accuracy, but they also have been both heavy and bulky. These prior neutron rem meters have used gas detectors with bulky and heavy moderators to monitor exposure to neutron radiation. These gas detectors rely on a gas such as boron trifluoride (BF3), which is excellent in detecting low-energy, or thermal neutrons having energies of approximately 0.025 eV. To obtain the high-energy response, however, the gas detectors must use a heavy polyethylene shell weighing approximately 20 pounds as a moderator to slow fast neutrons. This polyethylene moderator extends the gas detector""s response only to approximately 10 MeV.
Over the course of the last decade, moderator inserts of lead or tungsten have improved the high energy response of some detectors, but at a significant cost in terms of weight. These detectors can weigh 30 or more pounds, a weight that can result in muscle strain in the backs, arms, and shoulders of persons who repeatedly lift such detectors. Even with the lead or tungsten moderators, the high-energy response deficiencies of prior art detectors can cause these detectors to underestimate the neutron hazard in certain applications.
These gas-based detectors are also difficult and costly to maintain. The gas detectors have to be replaced every three years at significant expense.
Throughout the world, neutron rem meters are used by health physicists for real-time measurement of neutron dose equivalent. The neutron rem meter has become the instrument of choice in radiation fields in which the neutron spectrum is unknown or is poorly characterized. They also are useful in detecting and quantifying radiation hazards around nuclear reactors, accelerators, isotopic and fusion sources. However, the problems with prior art detectors, which have been outlined above, have limited the usefulness of most detectors in many applications. The bulk and weight of the prior art detectors have limited their application. The lack of high-energy response has rendered them of limited value in other applications.
The present invention provides a scintillator-based neutron rem meter that is light, compact, and responsive to high-energy neutrons. The present invention is designed to be a hand-held device. It weighs only about 4 pounds and is compatible with most neutron counters. The present invention operates under the principle of proton recoil, in which neutrons strike protons. The protons recoil and strike grains of scintillating material. This collision releases energy in form of optical photons.
It is therefore an object of the present invention to provide a neutron rem meter that is responsive to high-energy neutrons.
It is another object of the present invention to provide a neutron rem meter that is light in weight.
It is yet another object of the present invention to provide a neutron rem meter having uniform directional response.
It is still another object of the present invention to provide a neutron rem meter that is compatible with most current neutron counters.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.
To achieve the foregoing and other objects, and in accordance with the purposes of the present invention, as embodied and broadly described herein, a neutron rem meter comprises a lightguide defining a cylindrically shaped penetration, with a plurality of first moderators each of the moderators defining a central aperture and individually mounted to the lightguide. A second moderator defines a central aperture coaxial with the cylindrically shaped penetration in the lightguide and mounted to the lightguide. A plurality of fast neutron scintillators, individually mounted in the central aperture of each of the plurality of first polyethylene moderators, and a cadmium filter, a thermal neutron scintillator, a plastic spacer and a photomultiplier tube located inside the cylindrically shaped penetration in the lightguide.
In another aspect of the present invention and in accordance with its principles and purposes A neutron rem meter comprises a cubical lightguide defining a cylindrically shaped penetration and having a top, bottom and sides, with four first moderators each of the four first moderators defining a central aperture and individually mounted to the four sides of the cubical lightguide. A second moderator defines a central aperture coaxial with the cylindrically shaped penetration in the lightguide and mounted to the top of the cubical lightguide. A third moderator is mounted to the bottom of the cubical lightguide. Four fast neutron scintillators are individually mounted in the central aperture of each of the four first moderators, and a cadmium filter, a thermal neutron scintillator, a plastic spacer and a photomultiplier tube are located inside said cylindrically shaped penetration in the lightguide.